The introduction of the Scottish striker just after the hour added the bite the Bradford attack had been lacking. In the 75th minute he found the space 15 yards out to connect with Wayne Jacobs' cross from the left to head in off the upright.
"The game had 0-0 written all over it, but putting 'Flash' on was an inspired substitution," said the Bradford manager, Paul Jewell.
"Our quality was only shown in the goal. It was the only time we passed the ball all night," he added.
Something drastic was needed to lift the game, for there had been little to suggest that one of the sides had Premiership aspirations and the other had been in the top flight only a year ago.
Bradford's excuse was that they had the visit of Tottenham Hotspur for Saturday's FA Cup sixth round tie on their minds.
But even a greasy Oakwell surface was no excuse for the spate of misplaced, over-hit passes and miscued shots and clearances that punctuated proceedings.
The Barnsley striker Bruce Dyer did find space for a couple of shots, though they penetrated no further than Darren Moore's blocking boot, and Bradford's Lee Mills had a header cleared off the line by Nicky Eaden.
Bradford goalkeeper Gary Walsh palmed away an angled drive by Barnsley left-winger Scott Jones, and Craig Hignett wasted a couple of decent opportunities.
Mike Sheron had the ball in the net for Barnsley from Hignett's free- kick. But his celebrations were short-lived: his enterprise was ruled off-side - a decision his manager, John Hendrie, afterwards disputed.
But there was no arguing with Watson's goal, and on balance Bradford were just about worth their victory.
Barnsley (3-4-1-2) T Bullock; Morgan, Markstedt (M Bullock, 80), De Zeeuw; Eaden, Van der Laan, Tinkler, Jones; Hignett; Dyer, Sheron (Goodman, 86). Substitute not used: Blackmore.
Bradford City (4-4-2) Walsh; Todd, Dreyer, Moore, Jacobs; McCall, Whalley, Blake (O'Brien, 83), Lawrence; Rankin (Watson, 65), Mills. Substitute not used: Ramage.
Referee: G Singh (Wolverhampton).Reuse content